Bytecode: the block and non-block versions now coexist in parallel

As a consequence, a new “block_decomposed” option of the bytecode MEX has been
introduced to explicitly select the block-decomposed version.

Note that we do not always use the “block_decomposed” option even when the
“block” option has been passed to the user, in situations where the block
decomposition brings nothing (e.g. when evaluating the residuals of the whole
model).